The invention relates to a power and free conveyor comprising
a) at least one free rail;
b) at least one transport carriage, which comprises an, in direction of motion, leading front runner and an, in direction of motion, trailing rear runner, each of which is guided by the free rail, wherein front runner and rear runner each have a pusher dog, which may adopt a higher driving position and a lower position of rest;
c) at least one power rail, in which a power chain is guided, which carries a plurality of chain pusher dogs, which cooperate with the pusher dogs of the transport carriage in their driving position,
d) in at least one region of the path of motion of the a transport carriage the power chain deviates from the course of the free rail in such a way that the chain pusher dogs are no longer in engagement with the front runner pusher dog and in said region the transport carriage are moved through interaction between the chain pusher dogs and the rear runner pusher dog.
Power and free conveyors are two-rail systems. In the upper power rail a power chain with pusher dogs runs continuously. Running in the lower rail, the load or free rail, are the transport carriages, to which the load to be conveyed is attached. For specific processing operations of the loads attached to the transport carriages it is necessary to lower said loads in a special lifting station, which is introduced into the course of the free rail, and then lift them again. In the region of said lifting station it is impossible for the power chain to be conveyed parallel to the free rail situated below because space is needed for the cables, chains or belts, with the aid of which the load is moved vertically. For said reason, in said region the power chain swings out sideways before resuming its parallel course to the free rail in the end region of the lifting station. In the region where the power chain no longer extends above the free rail an interaction between the chain pusher dogs and the front runner pusher dogs of the transport carriages may no longer occur, thereby ruling out the front runner pusher dogs for moving the transport carriages. This is where the rear runner pusher dogs come in, which are still situated in a region, in which the power chain lies sufficiently precisely above the free rail. Through interaction between the chain pusher dogs and the rear runner pusher dogs of the transport carriages the latter are pushed so far into the lifting station that the front runner pusher dog may in principle already be grasped again by the chain pusher dogs.
Like the pusher dog of the front runner, the pusher dog of the rear runner also has to be capable of adopting two different positions, namely a position of rest, which lies below the path of motion of the chain pusher dogs, and a driving position, in which it projects into the path of motion of the chain pusher dogs. In known, commercially available power and free conveyors of the type described initially, the rear runner pusher dog is supported pivotally about a horizontal axis, wherein a suitable centre-of-gravity position ensures that the rear runner pusher dog has a tendency to rotate into the driving position. This is prevented in all locations, where the rear runner pusher dog is to adopt its position of rest, by the fact that the rear runner pusher dog lies with its uppermost point against a suitable guide surface of the free rail and slides along said guide surface. It is only in locations, where the rear runner pusher dog is to switch over into the driving position, that the guide surface of the free rail has cutouts so that the rear runner pusher dog may tilt further into the driving position.
Said known construction has several drawbacks: firstly, the guide surface of the free rail and the rear runner pusher dog are subject to extreme wear as a result of the constant sliding contact. Secondly, however, the positions, at which the switchover of the rear runner pusher dog into the driving position is to occur, are not variable but permanently defined by the cutouts in the free rail.
The object of the present invention is to construct a power and free conveyor of the type described initially in such a way that as little wear as possible occurs at the rear runner pusher dog and that the positions along the path of motion of the transport carriages, at which the switchover of the rear runner pusher dog is effected, are variably selectable.
Said object is achieved according to the invention in that
e) the two positions of the rear runner pusher dog are stable positions in the sense that for a change between the two positions a resistance has to be overcome;
f) switchover devices are provided along the path of motion of the transport carriage at the locations where a change of the positions of the rear runner pusher dog is to occur and, to said end, exert on the rear runner pusher dog a force, which overcomes the resistance.
The power and free conveyor according to the invention therefore renounces the old concept, according to which the rear runner pusher dog is constantly loaded in the direction of its driving position and movement into the driving position is prevented by sliding contact with the free rail at the appropriate locations. Instead, the rear runner pusher dog according to the invention is mounted in a xe2x80x9cbistablexe2x80x9d manner: it maintains each of its two operating positions permanently so long as the resistance between the two positions is not overcome through application of an external force. This means in particular that in the position of rest the rear runner pusher dog does not lie slidingly against any other part, with the result that wear is prevented. The switchover devices, by means of which the force needed to overcome the resistance between the positions of the rear runner pusher dog is applied, may be mounted on at any desired positions of the free rail. Subsequent adjustment is also easily possible. Thus, the power and free conveyor according to the invention is very much more variable than that according to prior art.
On the rear runner pusher dog a lug is preferably formed, which cooperates with a spring fastened to the rear runner. Upon pivoting of the rear runner the lug presses upon the spring, which between the two positions of the rear runner pusher dog experiences a maximum deformation but in the two stable positions of the rear runner pusher dog, on the other hand, relaxes or is only deformed relatively slightly.
When the spring comprises an offset end region, the point, at which the resistance between the two positions of the rear runner pusher dog is to be overcome, may be marked precisely.
The switchover device, which presses the rear runner pusher dog out of its driving position into the position of rest, is preferably a stop strip, which in a second function is intended for interaction with the front runner pusher dog.
Such stop strips are already known from prior art. They may be moved from a position outside of the path of motion of the front runner pusher dog into a position within said path of motion, in which position they press the front runner pusher dog of the approaching transport carriage down and hence bring it out of engagement with the chain pusher dogs of the power chain. By virtue of the fact that in the present embodiment of the invention said stop strip is assigned a second function during the actuation of the rear runner pusher dog, the number of necessary components may be kept low.
The switchover device, which presses the rear runner pusher dog out of the position of rest into the driving position, may be a crank part. By virtue of appropriate surfaces and/or parts of said crank part and of the rear runner pusher dog sliding past one another the rear runner pusher dog is brought into the driving position, there being no need for further control elements for said purpose.
To reduce the expenditure of force associated with the switchover of the rear runner pusher dog into the driving position, the crank part may cooperate with a roller provided on the rear runner pusher dog.